This new variable is extremely useful when iterating on creating a large package,
as otherwise you have to wipe the source and rebuild each time you make a mistake
with the patches or build.sh script.
Simply set TERMUX_PKG_QUICK_REBUILD=true in build.sh if a build fails and then the
TERMUX_PKG_SRCDIR and TERMUX_PKG_BUILDDIR will not be touched when you rebuild,
including that the patches will not be applied again. When you're done iterating,
diff for any new patches, save them, and remove this variable before rebuilding
from scratch, hopefully for the last time. ;)
An example is shown for the giant libllvm package, where other modifications are
also excluded if this variable is set.
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